Polarization and wavelength independent optical waveguide tap

ABSTRACT

An integrated optical device for tapping signal power provides a tap which is substantially independent of wavelength and polarization. The optical device includes a first tap consisting of a first optical waveguide carrying an input signal S disposed in coupling relation with a second optical waveguide for providing an output cross-state transmission T 1 . The cross-state transmission T 1  is polarization and wavelength dependent. The optical device further includes a second tap consisting of a third optical waveguide disposed in coupling relation with and in series with the second optical waveguide carrying the output cross-state transmission T 1  for providing an output bar-state transmission T 2 . The bar-state transmission T 2  has an opposite dependency on waveguide coupling than that of the cross-state transmission T 1  and, thus, compensates for wavelength and polarization dependencies of the cross-state transmission signal T 1 , thereby providing an overall tap transmission T which is equal to T 1  T 2  and is substantially independent of polarization and wavelength.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated optical devices and, moreparticularly, to an optical device contemplated for tapping signal powerfrom an optical waveguide whereby the tapped signal is substantiallyindependent of polarization and wavelength.

2. Description of the Prior Art

Due to the increase in the use of optical fiber communication channels,the development of integrated optical devices for directly processingoptical signals has become of significant importance to systemdesigners. One particularly useful approach for optical processing isthrough the use of integrated glass waveguide structures formed onsilicon substrates. The basic structure of such devices is described inC. H. Henry et al., "Glass Waveguides on Silicon for Hybrid OpticalPackaging" 7 J. Lightwave Technol., pp. 1530-1539 (1989). In essence, asilicon substrate is provided with a base layer of SiO₂ and a thin corelayer of doped silica glass is deposited on the oxide. The core layercan be configured to a desired waveguide structure-typically 5-7micrometers wide-using standard photolithographic techniques, and alayer of doped silica glass is deposited on the core to act as a topcladding. Depending on the precise configuration of the waveguide, suchdevices can perform a wide variety of functions including tapping ofsignal power from the optical waveguide.

In a typical signal tapping application of the aforedescribed integratedoptical devices, two waveguides are passed in close adjacency for alength, i.e., coupler length, dependent upon the desired degree ofcoupling. Energy from one waveguide core is transferred to an adjacentcore to effectuate the signal tap.

One shortcoming of such optical tap configurations is that the tappedsignal tends to be dependent upon the wavelength of the signal. Anothershortcoming concerns the birefringence induced in the waveguide by thestrain of the glass layers. The strain is due to the difference inthermal expansion of the glass films composing the waveguide and thesubstrate. It is compressive when the waveguides are formed on siliconsubstrates and it's magnitude varies with layer composition. Suchstrain-induced birefringence presents different indices of refractionfor the different polarization modes i.e., the transverse magnetic (TM)mode and the transverse electric (TE) mode of the transmitted light. Theeffect of this is that the mode confinement is polarization dependentand, consequently, the coupling of two waveguides becomes polarizationdependent. Thus, a tapped signal is provided, which is dependent on thepolarization state of the signal.

Several techniques have been suggested for overcoming the intrinsicbirefringence of glass-on-silicon waveguides. One method employs ahalf-wave plate inserted in the middle of a waveguide gratingmultiplexer to rotate the polarization by 90°. See H. Takahashi, et al.,"Polarization-Insensitive Arrayed-Waveguide Multiplexer on Silicon" Opt.Letts. 17(7), p 499 (1992). This approach, however, leads to excessiveloss. Another approach is to deposit on the waveguide a layer (sixmicrometers) of amorphous silicon. A drawback of this approach is thatthe silicon layer must be then actively trimmed with a high power laser.

Accordingly, there exists a need for further improvements incompensating for wavelength dependencies and strain-inducedbirefringence in integrated optical tap devices.

SUMMARY OF THE INVENTION

Generally stated, the present invention is directed to an optical devicefor tapping signal power wherein the signal tap provided issubstantially independent of wavelength and polarization. The preferredoptical device comprises first tap means including a first opticalwaveguide carrying an input signal S disposed in coupling relation witha second optical waveguide for providing an output cross-statetransmission T₁. The cross-state transmission signal T₁ is polarizationand wavelength dependent. The optical device further comprises secondtap means including a third optical waveguide disposed in couplingrelation with and in series with the second optical waveguide carryingthe output cross-state transmission T₁ for providing an output bar-statetransmission signal T₂. The bar-state transmission T₂ has an oppositedependence on the waveguide coupling than that of the cross-statetransmission T₁ and, thus, can be chosen to compensate for wavelengthand polarization dependencies of the cross-state transmission T₁ toprovide an overall tap transmission output T=T₁ T₂ which issubstantially independent of polarization and wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the invention will be betterunderstood from the following detailed description in conjunction withthe accompanying drawings wherein:

FIG. 1 is a schematic view of an integrated optical circuit includingthe substantially independent polarization and wavelength optical tap inaccordance with the principles of the present invention;

FIG. 2 is a cross-sectional view taken along the lines 2--2 of FIG. 1;

FIG. 3 is a graphical representation illustrating the theoreticalcalculated values of each of the tapped transmissions T₁, T₂ generatedby the two regions of the optical tap of FIG. 1 to produce an outputtransmission T which is substantially polarization and wavelengthindependent; and

FIG. 4 is a spectra of tapped transmissions T₁, T₂ and an overall outputT (where T=T₁ T₂) of an illustrative computer-simulated embodiment ofthe optical tap of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The following detailed description relates to a technique for tappingoptical energy from an optical fiber. This technique is based on thetheory that optical energy is transferred from one optical waveguide toan adjacent optical waveguide if the two waveguides are coupled in closeproximity for a certain distance or length, i.e., the "coupler length".

Referring now to FIG. 1, there is illustrated a schematic view of anintegrated optical device including the polarization and wavelengthindependent tap in accordance with the principles of the presentinvention. The illustrative embodiments of optical tap 10 described indetail hereinbelow are structured to be optically connected to a signalcarrying fiber or circuit for the purpose of sampling or monitoring thesignal propagating therethrough.

The tap 10 includes three waveguides disposed on a silicon substrate.Waveguide 12, which carries an input signal "S" from an optical signalcarrying waveguide, is disposed in coupling relation with waveguide 14at coupling region 16 for a coupler length "L₁ " to form a firstdirectional coupler which divides the light on the two waveguides intothe two outputs. Similarly, waveguide 14 is disposed in couplingrelation with waveguide 18 at coupling region 20 for a coupler length"L" to form a second directional coupler. Waveguides 12, 14 and 18 arepreferably identical with regard to geometrical and opticalcharacteristics. Similarly, the core center-to-center separationsbetween the respective waveguides at coupler regions 16, 20 areidentical. The respective coupler lengths L₁, L₂ may vary.

FIG. 2 illustrates the preferred method for fabricating tap 10. FIG. 2is a cross sectional view taken along the lines 2--2 of FIG. 1. Inaccordance with the preferred method, waveguide cores 12, 14 and 18 aredisposed at their appropriate positions upon a substrate 22 of siliconhaving an oxide outer layer 24 and then surrounded by a cladding layer26. The structure shown is advantageously fabricated as described in theaforementioned publication of C. H. Henry et al, "Glass Waveguides onSilicon for Hybrid Optical Packaging", 7 J. Lightwave Technol., pp.1530-1539 (1989). In essence, silicon substrate 22 is provided with abase layer 24 of SiO₂ formed on the silicon or by high pressure steamoxidation or by LPCVD (low pressure chemical vapor deposition). A corelayer of 6%-8% phosphorus doped silica having a thickness in the range4-6 micrometers is deposited on the oxide using LPCVD, and the corelayer can be dry etched as by RIE to pattern the waveguides to desiredconfiguration. The core glass is then annealed, and thereafter a 7micrometer layer of phosphorus and boron doped silica is deposited toact as a top cladding. Typical waveguide core widths are in the range5-7 micrometers.

Referring again to FIG. 1, the operation of optical tap 10 will bediscussed in detail. As a result of the coupling of optical waveguides12, 14 at coupling region 16, a portion of the signal "S" carried byoptical waveguide 12 is coupled onto waveguide 14. The tapped signalidentified as T₁ represents the cross-state transmission of the couplingand is wavelength and polarization dependent. Consequently, theresulting output signals at locations 16A, 16B are 1-T₁ and T₁,respectively, where 1 is substituted for signal "S" and representsunity.

At coupling region 20, a portion of cross-state transmission signal T₁is tapped onto waveguide 18. The resulting output signals at position20A, 20B are identified as T₁ T₂ and T₁ -T₁ T₂, respectively, where T₂is the bar state transmission effectuated by the coupling. TransmissionT₂ is also wavelength and polarization dependent.

The output signal T defined at position 20A may, under the appropriateconditions, be substantially independent of the polarization andwavelength of the respective transmissions T₁ and T₂. In particular, thecross-state output T₁ and the bar-state output T₂ will possess oppositedependencies on waveguide coupling. Consequently, a cancellation ofthese dependencies will occur, provided transmission T₂ is of theappropriate compensating magnitude, since the overall outputtransmission T at location 20A is the product of the cross state ofcoupler region 16, i.e., T₁, and the bar-state of coupler region 20,i.e., T₂.

It is possible to theoretically determine the value of transmission T₂which is required to compensate for the polarization and wavelengthdependencies of transmission T₁ so as to generate the substantiallypolarization and wavelength independent tap transmission T at theoutput. In this regard, it is to be appreciated that tapped signals T₁,T₂ are directly related to a coupling parameter (hereafter referred toas parameter δ) which is a function of the optical characteristics ofthe waveguide materials, center to center spacing dimensions of thecoupled waveguides, etc... For each coupler, the cross-statetransmission equals sin² (δL) and the bar state transmission equals cos²(δL), where L is the coupler length. Parameter δ is inversely related tothe coupling length which is the length for which the power wouldcompletely transfer between waveguides.

As stated above, the power transmission of tapped signal T at location20A is represented by the equation T=T₁ T₂. We will find the relationbetween T₁ and T₂ such that the derivative ##EQU1## T depends onwavelength and polarization through the dependence ofδ on strainbirefringence and wavelength. Thus, setting ##EQU2## also sets to zerothe first order (linear) dependence of T both on wavelength and strainbirefringence. Differentiating 1n (T)=1n (T₁ T₂), where 1n is thenatural logarithm, we find: ##EQU3## Thus, by setting ##EQU4## we cantheoretically find the values of tapped transmissions T₁ and T₂ suchthat T is insensitive to a change in δ, i.e., for a given value T₁, avalue of T₂ can be identified which compensates the wavelength andpolarization dependencies of T₁ and produces a tap output transmission Tsubstantially independent of polarization and wavelength.

The amplitudes of the waveforms of T₁ and T₂ are a function of δ and therespective coupler lengths L₁, L₂ at coupling regions 16, 20. As statedabove, coupler lengths L₁ and L₂ are defined as the effective lengthsassociated with coupling including the ends of the couplers where thewaveguides are bending away from each other. The transmission T₁ isrepresented by sin² (δL₁) and the transmission T₂ is represented by cos²(δL₂) where L₁ is the coupler length of coupling region 16 and L₂ is thecoupler length of coupling region 20. Thus,

    T.sub.1 =sin.sup.2 (δL.sub.1)                        (2)

and

    T.sub.2 =cos.sup.2 (δL.sub.2)                        (3)

The derivative of equation (2) is ##EQU5## and the derivative ofequation (3) is ##EQU6## By substituting equations (4) and (5) intoequation (1), equation (1) becomes ##EQU7## Multiplying this equation byδ and dividing by 2, we find ##EQU8##

By reference to equation (7), it is to be noted that for a relativelysmall value of T₁, δL₁ is nearly zero, cos(δL₁) is approximately 1 andsin(δL₁) is equal to δL₁. Thus, for a small value of T₁, equation (7)becomes: ##EQU9## Therefore, for a relatively small value of δL₁, δL₂and T₂ are each constant. Consequently, there exists a universal valueof T₂ which represents the compensating tap loss required to compensatethe polarization and wavelength dependencies of T₁ so as to produce atapped signal T which is substantially polarization and wavelengthindependent, providing T₁ is small compared to unity.

Equations (2) and (3) can be used to rewrite equation (7) in terms of T₁and T₂. The resulting equation is: ##EQU10##

Equation 9 can be solved numerically to identify the theoreticalcompensating values of signal tap T₂ required for given values of outputsignal T₁. The graph of FIG. 3 illustrates the theoretical values of T₂required to compensate for polarization and wavelength dependencies ofT₁. As depicted in the graph and by way of example, for a value of T₁,the required value of bar state transmission T₂ needed to compensate forthe polarization and wavelength dependencies of T₁ is between -3 db and-3.71 db. At values of T₁ from about -17 db to -40 db, T₂ reaches itsasymptotic or universal value of -3.71 db. Thus, an additionalattenuation of at most -3.71 db is required for cancellation of thepolarization and wavelength dependencies of tapped signal T₁. This is arelatively small price to pay to provide a tapped signal T independentof wavelength and polarization.

Referring now to FIG. 4, in conjunction with FIG. 1, an illustrativeembodiment of the optical tap 10 of the present invention is depicted.The curves in FIG. 4 are spectra of the individual transmissions T₁,T₂and the output transmission T of a computer simulated embodiment of thedevice of FIG. 1. In accordance with the simulated embodiment, signalcarrying waveguide 12 is coupled with waveguide 14 at coupling region 16for a length L₁ of about 173 microns so as to form a -15 db directionalcoupler at a wavelength of 1300 nm for TE polarization. Waveguides 14,18 are coupled for a predetermined length L₂ of about 1850 microns atcoupler region 20 to form a -2.4 db directional coupler which provides abar state transmission of about -3.7 db within coupler 20 at awavelength of 1300 nm for TE polarization. In this embodiment, lengthsL_(l) and L₂ are each defined as the distances for which the respectivewaveguides 12, 14, 18 are in parallel coupling relation in theirrespective coupling regions 16, 20. Additional parameters of thisillustrative embodiment include a waveguide core width of about 5.0microns and a core center-to-center separation of about 9.5 microns.

As illustrated in FIG. 4, the individual output transmissions T₁, T₂ areboth polarization and wavelength dependent. In particular, thetransverse electric (TE) (which is the electric field parallel to thesubstrate) polarization and the transverse magnetic (TM) (which is theelectric field perpendicular to the substrate) polarization havedifferent transmission losses for its respective transmission T₁, T₂.Similarly, the transmission losses also vary with the wavelength of itsrespective transmissions T₁, T₂. In the graph, the TE polarization andTM polarization for transmission T₁ are illustrated as solid line TE₁and dashed line region TM₁, respectively. The TE polarizations and TMpolarization for transmission T₂ are illustrated as solid line TE₂ anddashed line TM₂, respectively.

Referring still to FIG. 4, the resulting output transmission Teffectuated by transmission T₁ and compensating transmission T₂ issubstantially polarization and wavelength independent. The TEpolarization and TM polarization for tap 10 are illustrated as solidline 50TE and dashed line 50TM, respectively. As shown, the TE and TMpolarizations are substantially identical and remain constant having atotal transmission loss of approximately--19 db throughout a wavelengthof 1250-1350 nm. Thus, in this example, to accomplish the compensationof tapped transmission T₁ so as to produce a substantially polarizationand wavelength independent transmission T would entail approximately aminus 3.6-3.7 db additional attenuation as provided through bar statetransmission T₂. This is relatively minimal attenuation required toachieve wavelength and polarization independence.

It is to be noted that in accordance with the principles of the presentinvention, compensation for polarization and wavelength dependencies maybe achieved if couplers 16, 20 differ in cross sections and possessdifferent values of coupling parameter δ.

The present invention also has application in other waveguidetechnologies where the operation of the tap can be described by acoupling parameter δ that controls the wavelength and polarizationdependencies, i.e., where the cross and bar state transmissions are sin²(δL) and cos² (δL), respectively. For example, the principles of thepresent invention can have application with semiconductor waveguideintegrated optical circuits having Group III-V semiconductor waveguidematerials.

Further, the arrangement of optical tap 10 may also be used in tappingsignals from a conventional non-integrated optical fiber network, so asto produce a tapped signal transmission which is substantiallyindependent of wavelength.

It is to be understood that the above-described embodiment isillustrative of only one of the many possible specific embodiments whichcan represent applications of the principles of the invention. Numerousand varied other arrangements can be made by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A passive optical device for tapping signalpower, which comprises:first tap means for passively tapping signalpower from an optical waveguide, said first tap means including a firstoptical waveguide carrying an input signal S disposed in couplingrelation with a second optical waveguide for providing an outputcross-state transmission T₁ ; and second tap means for passively tappingsignal power from said second optical waveguide, said second tap meansincluding a third optical waveguide disposed in coupling relation withsaid second optical waveguide carrying said output cross-statetransmission T₁ for providing an output bar-state transmission T₂, saidbar-state transmission T₂ compensating for wavelength and polarizationdependencies of said cross-state transmission T₁ such that an outputsignal T-provided by said first and second tap means is substantiallyindependent of polarization and wavelength.
 2. The optical deviceaccording to claim 1 wherein said output cross-state transmission T₁ andsaid output bar-state transmission T₂ have opposite dependencies on acoupling parameter.
 3. The optical device according to claim 2 whereinsaid output transmission T is defined as the product of said outputcross-state transmission T₁ and said output bar-state transmission T₂.4. The optical device according to claim 3 wherein said waveguidescomprise doped silica overlying a silicon substrate.
 5. The opticaldevice according to claim 4 wherein said cross-state transmission T₁ hasa value ranging from about -3.0 db to about -50.0 db and wherein saidbar-state transmission T₂ ranges from about -3.0 db to about -4.5 db. 6.An integrated passive optical device for tapping signal power from asignal carrying waveguide, which comprises:a first optical waveguide forcarrying an input signal S; a second optical waveguide passively coupledwith said first optical waveguide carrying said input signal S forforming a first tap with an output cross-state transmission signal T₁ ;and a third optical waveguide passively coupled with said second opticalwaveguide carrying said output cross-state transmission signal T₁ forforming a second tap with an output bar-state transmission signal T₂ ;wherein a resulting output transmission T of said second opticalwaveguide is defined as the product of said cross-state transmission T₁and said bar-state transmission T₂ and is substantially independent ofpolarization and wavelength of said transmissions T₁ and T₂.
 7. Theintegrated optical device according to claim 6 wherein said first,second and third optical waveguides comprise doped silica overlying asilicon substrate.
 8. The integrated optical device according to claim 6wherein said first, second and third waveguides possess substantiallythe same widths.
 9. The integrated optical device according to claim 8wherein said first and second waveguides are spaced a firstpredetermined distance when in a coupled state and wherein said secondand third waveguides are spaced a second predetermined distance when ina coupled state and wherein said first and second predetermineddistances are substantially equivalent.
 10. The integrated opticaldevice according to claim 6 further comprising a substrate and acladding layer disposed on said substrate, said cladding layer beingdisposed about said optical waveguides.
 11. The integrated opticaldevice according to claim 10 wherein said substrate comprises silicon,said cladding layer comprises silica or doped silica and said waveguidescomprise doped silica glass.
 12. The integrated optical device accordingto claim 11 wherein said cross-state transmission T₁ has a value rangingfrom about -3.0 db to about -50.0 db and wherein said bar-statetransmission T₂ has a value ranging from about -3.0 db to about -4.5 db.13. A method for passively tapping signal power from a signal carryingwaveguide to provide a tapped signal which is substantially independentof polarization and wavelength, the method comprising the stepsof:passively coupling a first optical waveguide with a signal carryingwaveguide carrying an input signal S to a second optical waveguide toproduce a first tap with an output cross-state transmission T₁ ; andpassively coupling said second optical waveguide carrying saidcross-state transmission T₁ with a third optical waveguide to produce asecond tap having an output bar-state transmission T₂, said outputbar-state transmission T₂ compensating for wavelength and polarizationdependencies of said cross-state transmission T₁ such that an outputsignal T of the second optical waveguide effectuated by coupling of saidfirst and second optical waveguides and said second and third opticalwaveguides is substantially independent of polarization and wavelength.